Poor idle speed stability arising from changes in engine load, even small ones, has been recognized as a seemingly inherent operating characteristic of a basic diesel engine. Speed instability manifests itself by engine speed oscillating and/or wandering in consequence of a load change, rather than quickly stabilizing at a constant speed.
Various devices, special flyball governors for example, have been added to diesel engines in attempts to secure better speed stability. While some improvements have been made over the many years that diesel engines have been in existence, the inventors believe it is fair to state that none has been able to achieve complete success in overcoming this seemingly inherent and undesirable characteristic of such engines.
Control of engine idle speed in a governed diesel engine has been historically based on controlling the quantity of fuel introduced into each cylinder during the stroke of a piston that reciprocates within a cylinder, i.e. a fuel quantity-per-stroke basis. By their observation that a diesel engine is capable of operating at any of multiple different speeds using approximately the same fuel quantity per stroke, the inventors believe that a governing strategy that controls idle speed using strictly fuel quantity-per-stroke cannot provide an effective solution for idle speed control.
That a known idle speed governor embodying a known governing algorithm acting to control engine fueling via known devices and hardware is prone to instability when operating on a fuel quantity-per-stroke basis, is illustrated by the following situations.
If the idle speed governor is locked to a particular quantity of fuel per stroke in order to run the engine at a desired idle speed, any change that decreases engine speed, such as a change in engine load due to an engine-driven accessory being activated, will necessarily decrease the fueling rate to the engine. In other words, because the engine slows, there will be fewer strokes per unit of time while the quantity of fuel per stroke remains unchanged. That is exactly the opposite of what the engine actually needs in order to maintain desired idle speed, and consequently idle speed becomes unstable, at least temporarily.
If the idle speed governor is locked to that same particular quantity of fuel per stroke in order to run the engine at the same desired idle speed, any change that increases engine speed, such as a change in engine load due to the engine-driven accessory being de-activated, will necessarily increase the fueling rate to the engine. In other words, because the engine speeds up, there will be more strokes per unit of time while the quantity of fuel per stroke remains unchanged. That is exactly the opposite of what the engine actually needs in order to maintain desired idle speed, and consequently idle speed becomes unstable, at least temporarily.
While the advent of electronic control systems has yielded significant advances in diesel engine control technology and resulting engine performance, governing strategies have continued to rely on quantity-per-stroke as the basis for idle speed control. The evolution of electronic diesel engine control systems has resulted in the use of separate electronic modules for engine control and for fuel control, and their presence has created further complications for idle speed governing. An engine control module is sometimes referred to as an ECM, and a fuel control module as an ICM (injector control module), and although they are able to communicate with each other, each has its own separate processing system.
The use of separate ECM and ICM modules has placed added demand on the idle speed governor, tending to make stabilization of idle speed more difficult. This is essentially due to communications and scheduling delays between the different modules creating phase shift between the instant of time at which engine speed is measured and the instant of time at which a resulting fueling change can occur in consequence of a change in engine speed.
In any feedback control system, an electronic engine governor being one example, phase shift is commonly a limiting factor in tuning the gain of the control loop. Increasing phase shift tends to make the control less stable and ultimately unstable if the phase shift becomes too large.
The combination of the idle speed instability that is seemingly inherent in a diesel engine and the added phase shift resulting from the use of separate electronic modules is believed counterproductive to the objective of optimizing idle speed control in an engine governor. If the control loop gain is de-tuned to achieve stability, the engine responds poorly when engine load changes. If the gain is increased for better response, the system tends toward instability.
The inventors believe that a fundamental change in the strategy for control of the engine idle speed in a governed diesel engine is essential for attainment of the best possible way to optimize engine idle speed control.